This invention relates to a variable ratio speed control useful in various forms of belt and pulley drives, particularly where precise speed changes over relatively small differences are required. One particular demand for such devices is in magnetic recording devices of the type which transport magnetic recording media past a scanning record/playback transducer which is moved, as by a rotating disc or drum, to scan repeatedly across the path of movement of the media.
A number of prior art devices have been proposed to satisfy this particular demand. One such device, employing controlled belt slippage, is described in U.S. Pat. No. 3,006,218. Another controlled slip or migrating belt device, offered in some commercial video tape recorders uses a motor which drives the tape transport capstan at a constant speed, and which overdrives the scanner through an elastic belt, using an eddy current brake to slow the scanner to the desired speed. A third approach has been to use separate motors to drive the transport (capstan) and the scanner, and to regulate their respective speeds to the desired ratio by electronic comparators and motor speed controls.
Where portable recorders present strict limitations as to size, weight, and/or power consumption these approaches are unsatisfactory. Belt slipping wastes power, extra motors add weight and require extra power for both drive and speed controls, and continually active slip controls (eddy current brakes) need continuous additional power in addition to wasting power with induced slipping.
Even where some or all of the aforementioned design limitations are not present, there exists a need for an accurate, predictable, minimum power consumption speed change device. This is especially for such devices which can easily be linked to servo systems, and which will consume only minimal power during speed changing mode, and not during steady running conditions at different speeds.